Malvern Instruments GmbH

Fully automated particle size measurement technology integrated with existing plant control platforms for ease of use has the potential to deliver numerous benefits. At plants where a switch to on-line analysis has been made it is frequently easier or possible to:

Control the plant more effectively during steady state and transient operation either by improving manual control or switching to automatic control

Fully understand the interactions between different process parameters and/or carry out systematic studies to fully optimize the process

More consistently manufacture product with the required specification

Minimize certain variable costs - such as those associated with waste or energy consumption during size reduction

More rapidly identify process upsets hence minimizing their impact

Many stable processes can be accurately tracked and controlled by taking samples every couple of hours from the process for an off-line particle size analysis in the laboratory. However for unstable processes or those processes where huge rewards and significant increases in bottom line profit can be generated, on-line analysis can be highly beneficial.

On-line analysis provides a continuous stream of data to the PLC, and ensures that process behaviour can be fully observed and acted upon in a timely fashion by closing the loop.

For a number of years, on-line analysis and the automated control that it facilitates has provided an alternative to off-line analysis and the manual control which typically accompanies it.

For solids processes, particle size is frequently the key variable and therefore on-line analysis can be highly beneficial. For dry solids handling processes, in industries as diverse as pharmaceuticals and cement production, on-line particle size analyzers based on laser diffraction technology have been used successfully for many years. They generate significant cost benefits in the form of improved process efficiency and enhanced product quality.

More recently, new developments have made on-line laser diffraction technology accessible for wet processes also. Laser diffraction particle size analyzers can now be used reliably for a range of wet systems, from emulsions to highly concentrated slurries. Benefits similar to those enjoyed by dry processors are now being realized.

Why laser diffraction?

A variety of different technologies can be used for wet process particle size analysis, all of which have different strengths and weaknesses.

Laser diffraction (or low angle laser light scattering) is an attractive technique in that it rapidly generates consistent volumetric particle size analysis without the need for any external calibration. It is non-destructive and robust in terms of ambient conditions. Using modern systems, particles across a broad size range, typically 0.5 to 1000 m for wet systems, can be measured accurately. Its drawback is that it requires the media to be (to some degree) transparent; unless a certain amount of laser light can penetrate the sample, analysis is impossible. It is primarily this constraint that has previously limited the use of laser diffraction technology in a range of wet applications on-line. However, by developing mathematical algorithms to extend the concentration range over which laser diffraction can be used, and designing effective sample extraction and preparation systems capable of producing from a concentrated slurry a representative sample stream appropriate for analysis, the applicability of the technique has been significantly extended.

It is easy to appreciate that calculating particle size distribution from the diffraction pattern is straightforward if the light is scattered only by a single particle as it passes through the sample. However, if it is only samples of this type that can be analyzed then the applicability of the technique is constrained to highly dilute streams. A further refinement of the technology has involved the development of algorithms which can take into account multiple scattering (when light is scattered by several particles before reaching the detectors). Instruments with this refinement are capable of analyzing far more concentrated samples. This is an important development for wet applications as it reduces the amount of dilution required on concentrated samples.

Studies have shown that for reliable measurement by laser diffraction, a minimum of 5-10% of the laser light needs to pass through the sample; this typically equates to a solids content of 0.1-1.0% by volume. For many applications therefore sample dilution is an important step. In addition, other conditioning processes may also be required, for example additives or ultrasonics may be used to prevent particle agglomeration, or break up aggregates.

Sample extraction and dilution solutions for wet samples

For free-flowing liquid systems sample extraction from the process can be achieved using a simple eductor but for more demanding slurries, more complex systems are required.

Static sampling/Tank dilution

Mineral processors typically need to sample and analyze high tonnage, concentrated slurry streams with specific gravities in the range 2-8. For many applications in this industry a two-stage sampling procedure, in combination with dilution in a continuous stirred tank, has proven highly effective.

Rotating sampler/diluent powered diluter

If continuous sample extraction poses a significant challenge and sample dilution is required a rotating type sampler, similar in design to a four-way valve, can be used to extract slugs of flow from a process stream from which a continuous sample stream could not be successfully removed; the resulting system can still provide continuous on-line measurement.

Proof efficient dilution by measurement results:

An effective sample dilution system needs to dilute the sample representatively, be highly reliable and preferably not require significant manual intervention for maintenance, operation, or cleaning purposes. The Insitec diluter is mechanically relatively simple, for trouble-free operation, and has no moving parts. The throat and tip of the unit are designed such that the diluent entrains a slurry sample through into the central tube. Diluted sample flows through the tube to further identical dilution stages. At each stage the throat, which effectively forms a venturi mixer, ensures rapid intermixing with the clean diluent, thereby delivering a homogeneous sample to later dilution stages and ultimately the analyzer. The diameter of nozzle used in the design dictates the dilution ratio, which can be controlled in the range 2:1 to 5:1. The unit is suitable for particles with a maximum diameter no greater than 150 microns.

The key test of any diluter is the consistency of particle size measurements made at different dilutions. Figure 1 shows data for a talcum slurry measured using the diluter discussed above. The extent of dilution has no impact on particle size, confirming the representative nature of the dilution process and the effectiveness of the design.

Fig. 1: Data from on-line analyzer monitoring homogenizer product.

Optimize Emulsion production by controlling particle size online

Homogenizers are widely used within the food, dairy, cosmetic and pharmaceutical industries to produce emulsions with the required droplet size and hence the desired properties; droplet size impacts directly on product taste, consistency, performance and stability. Figure 2 shows data from an on-line laser diffraction analyzer monitoring an emulsion being produced using an APV homogenizer.

For this application no sample preparation is required and the emulsion is simply routed through the analyzer using a liquid flow cell. The sample is analyzed in its produced state thereby avoiding the problems with coalescence and aggregation that can occur when an emulsion sample is removed from the process for off-line analysis. The data shown are the result of a study of the effect of homogenizer operating pressure on droplet size. The four observable regions (blue trace) correspond to four different sets of operating conditions. The study time period is 20 minutes and the effect of the changes in operating conditions on Dv(50), or average droplet size, are easily observed.

With off-line analysis this type of study is time-consuming and prone to error as a result of, for example, operator variability and sample stability. With on-line analysis, however, the effects of processing variables on droplet size are rapidly determined and optimal conditions more quickly identified. This type of systematic study can be carried out during product development, or at the beginning of a production run to optimize the processing response to a change in, for example, feed material quality. In either the case the rapid identification of optimal operating conditions and the accompanying development of improved process knowledge, lead directly to better manual or automatic control, enhanced process efficiency and variable cost savings.

Conclusions

Hardware solutions for the sampling and presentation of difficult to handle slurries, coupled with the development of algorithms which allow for multiple scattering, have greatly increased the applicability of laser diffraction analysis. Manufacturers operating wet processes, across a range of industries, are now able to access this key on-line particle sizing technology. Opportunities for on-line analysis and automated control, and the huge benefits which they bring in terms of manpower savings, operating consistency and safety, are growing for solids processors.

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